[gimp/gimp-2-10] Issue #5008: Always true condition (i>=0) inside fill_inverse_cmap_gray.

[Jehan <jehanp src gnome org>]

commit ac7e841e241c553371550d4604bd850406f018b6
Author: Jehan <jehan girinstud io>
Date:   Sat Apr 24 14:43:43 2021 +0200

    Issue #5008: Always true condition (i>=0) inside fill_inverse_cmap_gray.
    
    Removing useless condition, add a g_return_if_fail() assertion for the
    only (impossible unless bug) case which we don't expect. Also set
    default mindist to G_MAXLONG instead of a magic number (which was ok now
    but might become a problem if some day colormap allowed more than 16-bit
    per channel colors).
    Finally break when we reach a distance of 0 since we won't get lower
    anyway, so better stop early.
    
    Thanks to Rafał Mikrut and Øyvind Kolås for code commenting.
    
    (cherry picked from commit d6dac255790d6004a3df0fa93f6fc7bbdbf3d3d6)

 app/core/gimpimage-convert-indexed.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/app/core/gimpimage-convert-indexed.c b/app/core/gimpimage-convert-indexed.c
index 89347a7bb9..3ba9d19bf9 100644
--- a/app/core/gimpimage-convert-indexed.c
+++ b/app/core/gimpimage-convert-indexed.c
@@ -2688,7 +2688,9 @@ fill_inverse_cmap_gray (QuantizeObj *quantobj,
   gint   mindisti;
   gint   i;
 
-  mindist  = 65536;
+  g_return_if_fail (quantobj->actual_number_of_colors > 0);
+
+  mindist  = G_MAXLONG;
   mindisti = -1;
 
   for (i = 0; i < quantobj->actual_number_of_colors; i++)
@@ -2699,11 +2701,13 @@ fill_inverse_cmap_gray (QuantizeObj *quantobj,
         {
           mindist  = dist;
           mindisti = i;
+
+          if (mindist == 0)
+            break;
         }
     }
 
-  if (i >= 0)
-    histogram[pixel] = mindisti + 1;
+  histogram[pixel] = mindisti + 1;
 }